Radar systems are well-known in the art, and can generally be divided into pulse radar systems and continuous-wave (CW) systems. A pulse radar system measures a distance to an object (usually referred to as target) by transmitting a short radio-frequency (RF) pulse to an object, and measuring the time taken for the reflected pulse (i.e. the echo) to be received. As the velocity of the pulse is known (i.e. the speed of light), it is straightforward to calculate the distance to an object. However, pulse radar systems are not suitable for use measuring distances of a few 100 meters, in particular because the pulse length must be reduced as the travel time (i.e. distance to the target) decreases. As the pulse length decreases, the energy contained within the pulse decreases, to the point where it becomes impossible to detect the reflected signal. Instead, continuous-wave radar systems are used for measuring comparably short distances. In many applications, such as in automotive applications, so-called frequency modulated continuous-wave (FMCW) radar systems are used to detect targets in front of the radar device and measure the distance to the target as well as their velocity.
Different from pulsed radar systems, in which isolation between the transmit signal path and the receive signal path is not specifically relevant due to the pulsed operation of the transmitter, a phenomenon referred to as leakage is an issue in FMCW radar systems. Leakage generally describes the problem that a small fraction of the frequency-modulated transmit signal “leaks” into the receive signal path of the radar transceiver without being back-scattered by a target. If the cause of the leakage is in the RF frontend of the radar transceiver (i.e. imperfect isolation of the circulator, which separates receive signal and transmit signal in a monostatic radar configuration) leakage is also referred to as crosstalk between the transmit signal path and the receive signal path. When integrating the radar system in one single monolithic microwave integrated circuit (MMIC) crosstalk or so-called on-chip leakage is always an issue.
Another cause of leakage may be objects, which are very close to the radar antenna (such as, e.g., a fixture or a cover mounted a few centimeters in front of the radar antennas). Herein, reflections of the transmitted radar signal at such objects are referred to as short-range leakage, which is a fraction of the transmit signal emanating from the transmit antenna and reflected back (back-scattered) to the receive antenna of the FMCW radar system at the mentioned objects, which are very close to the radar antenna(s). It shall be understood that the transmit antenna and the receive antenna are physically the same antenna in monostatic radar systems. Herein, the mentioned reflections caused by short-range targets are referred to as short-range leakage as their effect is similar to the effect of on-chip leakage. However, known methods, which are suitable for the cancellation of on-chip leakage or cross-talk are not suitable for the cancellation of short-range leakage.
In radar systems the overall noise floor limits the sensitivity, with which radar targets can be detected, and thus also limits the accuracy of the distance measurement. Generally, this noise floor is dominated by the additive noise of the transmission channel. However, in case a short-range target reflects the transmitted radar signal with comparably high amplitude (i.e. causes short-range leakage) the phase noise (PN) of the transmitted radar signal may dominate the noise floor. The phase noise results in a deteriorated signal detection quality or even makes the detection of radar targets with small radar cross sections impossible.